Tinplate has been successfully used for packaging foodstuffs by virtue of a fortuitous redox-potential reversal. Under normal oxygenated conditions, a steel substrate is anodic to the tin coating and any porosity of the tin coating exposes the steel to the unfortunate predicament of being a small anode-connected-to-a-big-cathode, leading to rapid red-rusting under atmospheric exposure, or pinhole perforation when exposed as a foodstuff package, i.e. a tin can. However, assuming an initial pore-free tin coating, the oxygen in a newly enclosed tin can (oxygen would be found in the headspace and be dissolved in the foodstuff) would be consumed by corrosion of the exposed free-tin during the stoving process in which the food can is heated, cooking the contents. This etching of the free-tin may expose the electrolytically inert FeSn2 intermetallic layer, an interfacial layer formed by reaction of the molten tin with the steel substrate during the reflow-melting process used in tinplate manufacture, or expose the steel substrate, if the FeSn2 intermetallic layer is porous, leading to can failure. Assuming no porosity of an exposed intermetallic layer, the free-tin corrosion continues until the can is fully de-oxygenated, at which point the electrochemical couple between tin and steel reverse: the steel substrate is now cathodic. Any steel then exposed by slow dissolution of the tin coating is cathodically protected by corrosion of the surrounding free-tin, resulting in a can life determined by the amount of available free tin (i.e. that not bound in the inert FeSn2 intermetallic layer).
The overall suitability and quality of tinplate for food packaging is thus determined by:                The gross porosity of the tin coating; the steel substrate should not be exposed during stoving, thereby forestalling rapid initial can failure.        The impermeability of the FeSn2 intermetallic layer. This layer should be dense and pore-free.        The amount of free tin higher amounts both allow for greater oxygen-removal during stoving and also permit long shelf-lives under deoxygenated conditions.        
Porosity is the principle factor in determining tinplate corrosion resistance, both bulk porosity and porosity of the FeSn2 intermetallic layer. Porosity of tinplate is commonly due to either sub-optimal tin electrodeposition, poor blackplate (the unplated steel substrate) activation (cleaning and pickling) and also gross surface inhomogeneities such as carbon residues, oils, oxide or other steel substrate inclusions.
There are different grades of tinplate, normally differentiated by their tin coating-weight, with lighter weights<2.8 g/m2 being used in an epoxy-lacquered state and thicker coatings>5.6 g/m2 being used in applications requiring superior corrosion resistance such as white-fruits such as pineapple, asparagus, or sulfide-stain resistant meats. For corrosion resistance, the impermeability of the intermetallic layer plays a role and tinplate manufacturers aim for impermeability by producing thicker intermetallic layers. As the FeSn2 intermetallic layer is formed by consumption of the free tin, a thick FeSn2 intermetallic layer typically requires a high initial tin coating weight and an extended reflow-melting time.
With the rising price of tin metal, tin packaging has been shifting to lighter coating weights in an effort to reduce cost, but have hit a technological barrier in that electrolytically produced tinplate for food packaging is traditionally considered to be porous at coating weights below 2.8 g/m2. Certain foodstuffs rely on the dissolved tin for further preservation of color, texture and flavor and cannot easily switch to a lacquered can. There thus exists a need for thin-tin and thin-tin alloy tinplate that has corrosion resistance equal to that of higher coating weight material.